490 PROCEEDINGS OF THE AMERICAN ACADEMY 



while the plates marked a on the octahedral face are parallel to a 

 lateral edge, and, on following the plates on to the face at right angles 

 to the first, it will be seen that they continue to be parallel to the 

 lateral edge. Hence they cannot be octahedral plates, and, since they 

 are parallel to a principal section of the octahedron, they must be 

 cubic. In order to see whether the Robertson County iron was an 

 unusual case of cubic plates, other well-marked Widmanstiittian irons 

 were examined and in the same way the cube was found, together with 

 the dodecahedron, in the De Kalb iron before mentioned, and also in 

 the Obernkirchen or Oldenburg iron. Undoubtedly, many other exam- 

 ples could be found, provided the proper faces could be distinguished 

 and etched. 



Thus it is evident that, in the first place, there is a regular unbroken 

 gradation between the coarsest Widmanstiittian figui'es and the finest 

 Neumann lines ; so that, beginning with the Nelson Co. iron, we can 

 arrange a series consisting of the irons of Wichita Co., Glorietta, Red 

 River, Robertson Co., Dickson Co., Oldham Co., Jewell Hill, Obern- 

 kirchen, Tazewell, Butler, Walker Co., Coahuila, and Hauptmannsdorf, 

 presenting regular gradations in which there is no gap where a definite 

 line of demarcation can be drawn. 



Moreover, there is no difference in crystalline form even so slight 

 as that between the three fundamental forms of the isometric system. 

 For, as has been shown, the coarsest Widmanstiittian figures as well 

 as the finest Neumann lines are intersections of planes of crystalline 

 structure, which may be parallel to faces of the octahedron, the do- 

 decahedron, or the cube. 



Of course it is not maintained that there is not a marked distinction 

 between characteristic Widmanstiittian figures showing all the features 

 of the three kinds of iron, and typical Neumann lines as exhibited by 

 the Hauptmannsdorf meteorite, but only that both are an outgrowth 

 as it were of the same type of crystallization. 



The Germans strongly insist on the divisions of the trias of Rei- 

 chenbach, and describe the features of kamacite, taenite, and plessite 

 as if they were essentially diiFerent substances, instead of merely 

 different conditions of a nickeliferous iron ; and the only evidence we 

 have that there is any msiterial difference of composition is based on 

 an analysis cited by Reichenbach* of the so-called taenite plates, 

 which he had mechanically separated from the Cosby Creek iron, and 

 which were found to contain 13.8 per cent of nickel, while the mass 

 as a whole only contained 9.8 ; and, on this ground, it is assumed that 



* Poggendorff s Annalen, Bd. cxiv. p. 258. 



